I. FIELD OF THE INVENTION
This invention relates to a process for the formation of silicon carbide and carbides of various metals, for example aluminum, boron, tantalum, titanium, zirconium, hafnium, yttrium, lanthanum, chromium, niobium, molybdenum, tungsten and vanadium, etc. More particularly, the invention relates to a process for producing such carbides in the form of fine powders or whiskers.
II. DESCRIPTION OF THE PRIOR ART
Carbides of silicon and various metals are well known and are generally characterized by high hardness, which makes them useful in a variety of applications. For example, tungsten carbide is used in large quantities for the preparation of cutting tools. Recently, interest has focussed on the production of carbides in the form of fine powders or whiskers, which are now being used, for example, for the fabrication of sintered bodies and components in the electronics and engineering industries and for the reinforcement of high melting point materials, such as ceramics, aluminum and other metals.
Tungsten carbide is commonly prepared by starting with tungsten oxide, reducing it with H.sub.2 and then reacting it with carbon black to produce the carbide. It has not, however, been possible to produce microfine powders and whiskers in this way.
Silicon carbide is traditionally formed by reacting silica and carbon in an arc furnace (Acheson process). Temperatures well over 2000.degree. C. are employed and the silicon carbide product is formed as large lumps and is quite impure. To form a fine powder, the large lumps have to be ground to the desired particle size, but the extreme hardness of silicon carbide makes the grinding step difficult and expensive. Recently, a-Sic has been produced by heating mixtures of fine or colloidal silica and fine carbon. However, as the distribution of the reactants is essentially non-homogeneous, the product is not very pure. A further way of producing silicon carbide in the form of a fine powder involves reactions between a silicon halide and a hydrocarbon, or the degradation of polycarbosilanes. However, the production costs of such methods are extremely high compared to conventional solid-solid reactions.
Silicon carbide whiskers are also produced commercially from rice hulls. Rice hulls contain SiO.sub.2 distributed in a carbonaceous matrix. This SiO.sub.2 is converted to SiC at high temperature, but both the distribution and the amount of the SiO.sub.2 present in rice hulls varies considerably and therefore the product quality is not controllable.
U.S. Pat. No. 4,444,894 issued on Apr. 24, 1984 to Robert G. Shaver illustrates a further method of forming silicon carbide, although it is the object of the patent to produce shaped articles made of a composite of carbon and silicon carbide rather than to produce fine silicon carbide powders. In this method, pulverized silica or hollow glass spheres are dispersed in a phenolic resin, the mixture is heated to high temperature in an inert atmosphere to carbonize the resin, and then the heating is further continued to cause the silica or glass and carbon to react to form silicon carbide. Hydrofluoric acid is then used to dissolve away unreacted silica or glass. However, the Shaver process does not form SiC in high yield (the conversion is generally of the order of 50%). Thus unreacted oxide remains present in fairly large amounts, and SiC particles of the desired degree of fineness and purity are not produced even if the unreacted carbon is removed.